1. Field of the Invention
This invention relates to a method for preserving aqueous solutions or dispersions containing organic substances which are degraded by microorganisms present as impurities. The invention also relates to a plant for carrying out this method and to applications of the method.
2. Description of the Related Art
There are a number of materials which must be held in aqueous solution or dispersion to facilitate their production and handling, their transport and storage as well as their application by the consumer. Such materials or products are required in particular in branches of industry like the paper and pulp industry. They include pigments, fillers, paper machine broke, starch, glue or retention agents, for example, which can all be prepared in the form of aqueous solutions or dispersions.
Microorganisms which are always present as contamination find optimal living conditions in this aqueous milieu. Owing to the high organic and inorganic nutrient offer and a favorable milieu with respect to temperature and pH value, such microorganisms show increased and quick growth.
Such solutions or dispersions always also contain other organic substances which are imported with the raw materials during preparation of the aqueous solutions or dispersions. Starch and glue as well as the composition of paper machine broke themselves constitute food for certain species of microorganisms.
With the increased growth of such microorganisms the organic substances present as contamination are increasingly degraded at the same time, which produces a change in the product-specific properties of the liquid product.
There are essentially two different conditions under which these aqueous liquid products are usually handled and which have a strong influence on the type of microbial growth and thus the microbial activity.
Under the first condition the liquid products are mixed continuously or at regular intervals under aerobic conditions. The air or oxygen feed this involves promotes the growth of particularly aerobic microorganisms. Such conditions are found particularly in systems where starch or glue are produced, or in broke pulpers and in the storage tanks or transport tanks for such products. One consequence of the increasing microbial contamination here is slime formation and deposit and a change in the desired specific product properties like viscosity, pH value, color and brightness.
Under the second condition the aqueous liquid products are not stirred but stored in tanks largely sealed off from air or oxygen. After a short initial phase in which the oxygen introduced during filling is consumed by the existing microorganisms, anaerobic microorganisms grow here. Owing to their metabolism these anaerobically respiring microorganisms release hydrogen ions which reduce nitrate, sulfate, sulfur and carbonates into nitrite, hydrogen sulfide, methane, acetic acid or other highly reduced compounds (Basic Biotechnology--A Student's Guide, P. Prave, U. Faust, W. Sittig, D. A. Sukatsch, VCH Verlagsgesellschaft Weinheim, 1987, page 101).
Anaerobically respiring microorganisms can therefore cause serious problems in particular with regard to the transport and storage of liquid products.
As already mentioned, these microorganisms release hydrogen ions which for their part have a reducing effect on certain substances which are either part of the liquid product or contained therein as contamination. For example, the reduction of sulfate or sulfur gives rise to sulfide in the form of hydrogen sulfide. Hydrogen sulfide is not only highly toxic, it is also more than undesirable because of its smell of rotten eggs and its property of discoloring products. Other reduction products like nitrite, methane or acetic acid also lead to an undesirable change in specific product properties, for example the brightness and smell of broke or crude fiber suspension, and the color and smell of finished paper.
To prevent, or at least check, the increase in microbial contamination and the related degradation or breakdown of organic substances present in the liquid products, few possibilities were available up to now.
One possibility is to use up the solution or dispersion very quickly before microbial degradation begins. This approach is only applicable to a limited extent since microbial degradation already sets in after a few hours under the usual optimal conditions. This possibility is practicable for example in connection with broke which is processed quickly, but totally unsuitable for example for conserving the stability of pigment slurries.
A further possibility is to store the aqueous liquid products under conditions which do not permit growth of the microorganisms. Such conditions are for example very high or very low temperatures. Both are very difficult to attain under practical conditions, and are most likely to be found in a starch digester or in storing starch.
A further possibility is to suppress, or completely eliminate, the growth of microorganisms by adding biocides. But only a limited number of biocides are effective against anaerobically respiring, in particular sulfate-reducing, microorganisms, and the latter are regarded as the worst form of contamination. Furthermore, the use of biocides meets increasingly with discredit. Although many of the biocides used in the paper industry are degraded very quickly, they are nevertheless dangerous for man and the environment if they go outside their normal area of use, for example due to a transport accident. Also, microorganisms are very likely to develop resistance to biocides and this can only be avoided by frequently changing the biocide or increasing the dose. Some biocides also contain halogens and thus contribute to the potential influence of absorbable organic halogens (AOX) on the environment. Many paper and pulp mills furthermore have biological clarification plants which can be greatly affected by improper handling of biocides.
A further possibility of controlling the growth of microorganisms in liquid products is to add strong oxidizing and bleaching agents. Oxidizing agents like hydrogen peroxide act very reliably, but their handling is very problematic not least because of the danger of corrosion. Such products are furthermore expensive, and treatment must take place continuously since the hydrogen peroxide is exhausted quickly. The same holds for bleaching agents like hypochlorite and bromochlorohydantoin. The latter furthermore contribute to increasing the AOX values in the finished product.
The problem of the present invention is therefore to provide a non-polluting method for preserving aqueous solutions and/or dispersions which is industrially applicable and reliably prevents microbial degradation of organic substances contained in these liquid products. The idea of the invention is to prevent the growth of such microorganisms by depriving them of their basis for food.